Innovative rehabilitation of a deep gulley

Gulley in 2017

Inside a section of the gulley in February 2019

An aerial view taken in September 2020, showing the extent of the erosion

The gulley just as construction began in 2020

Vergelegen Wines in Somerset West is very proud of its exposed and eroded rapidly. Between 2017 farm’s heritage and takes great care of the environment. and 2018, the gulley increased from 30 m Among other things, at its own cost, the company has to 400 m in length, and from 2 m to 6 m deep. Downstream from the gulley, an area cleared more than 2 000 hectares of alien vegetation of Lourens River fynbos was smothered by to create a fynbos reserve. One of the most recent the sediment. interventions entailed a major erosion control project. Plans for rehabilitation using cascades of By Hans King, Pr Eng Client: Vergelegen Wines Project design and management: HansKingSRS F ollowing the 2013 storm season, the appearance of a small gulley on the Vergelegen Wines farm was cause for weirs and other measures were prepared in early 2018 and the lengthy environmental approval process was started. An attempt was made to have the gulley recognised as a disaster and shorten the approval process – but, because the damage began in 2013,

Construction: Martin & East concern. From 2017, this gulley suddenly this was not allowed. The approval process

Year construction completed: 2021 accelerated in size as the dispersive took nearly a year, while the gulley grew weathered Cape granite soils below were even longer and deeper.

Technical challenges

The first challenge to the design was the wetland specialist’s requirements. The natural water table in the area had to be restored as far as possible to its original level, so that natural vegetation in the areas adjacent to the gulley would not be negatively impacted by the drying out of the soil. The implication of this was that the floor of the rehabilitated gulley had to be as close to the original ground level as possible, and this necessitated the construction of weirs 5 m to 7 m high – a potentially very costly exercise.

A serious challenge to the stabilisation of the gulley was the nature of the subsoil. Being highly dispersive, water flowing over it erodes it very quickly. To address this, steps had to be taken to make sure that water did not flow over the exposed, decomposed granite soil. In addition, the flow velocity of water had to be kept low so that the topsoil placed on top of the weathered granite was unlikely to be washed away.

At the design stage, there was an understanding that the depth of the gulley might well increase by the time the necessary approvals had been obtained and a contractor was finally on-site. The design had to be adaptable so that it could accommodate changes to the gulley shape.

The solution implemented

A cascade of seven gabion weirs was planned to reduce the slope of the gulley from 6% to around 1.5%, thereby reducing the flow velocity. Between the weirs, the gulley was widened to 4 m as a primary measure to reduce the flow velocity in the channel. To further increase the hydraulic surface roughness (and reduce flow velocities), it was planned to cover the channel with an assortment of indigenous wetland plants. These plants would have the added benefit of binding the soil with their roots.

Novel ideas

To drastically lower the construction cost of the gabion weirs, the gulley under the weirs was first filled with cement stabilised soil, so that most of the weir spillways only had to be 1 m high. Traditionally, the weirs would have been constructed from the base of the gulley floor and the cost of such structures would have made the project unaffordable. This cement-stabilised soil had the added advantage of being able to adapt to the shape of the gulley – which a built structure could not do. Trials were done using the decomposed granite and varying cement concentrations to determine a mix suitable for the project. Steps were taken to ensure that the topsoil layer spread over the

A perspective of the site taken in April 2021, prior to revegetation Construction in progress during October 2020

A Multi-Cell section being filled with topsoil

weathered granite base was not washed away before the vegetation could be properly established. A 150 mm thick Kaytech Multi-Cell was placed over the formed channel and the topsoil was placed into the Multi-Cell. A trial onsite, conducted prior to the finalisation of the design, proved that the MultiCell retained more than 90% of the soil placed in it when subjected to flow velocities of 2 m/s.

To achieve maximum support to the water table, the level of the weirs was planned so that there was just enough clearance between the spillways and the natural ground level for the expected floods to remain in the depression. This resulted in the depression between the weirs remaining between 1 m and 2 m below the natural ground level.

From an overall project perspective, the final objective was achieved thanks to an enduring design that blends in and coexists with the environment.